Osteoporosis is a major health concern for the elderly and has emerged as a significant public health problem. The majority of past research on this condition has been directed toward a thorough understanding of the disease process, its etiology, and its pathology. Research on the primary prevention of osteoporosis has lagged behind other areas of study. This dearth of effort is due, in part, to our previous lack of ability to non-invasively detect and accurately monitor changes in the skeleton. Methods and techniques developed and refined over the past 5 years now make it possible to monitor the effects of various preventive and treatment interventions. One viable strategy for the prevention of osteoporosis and its associated fractures is the use of systematic exercise and/or calcium supplementation aimed at building and maintaining peak bone mass. We propose to determine if a program of aerobic exercise in combination with calcium supplementation can enhance bone mass in a young premenopausal population. We will likewise assess the potential benefits of exercise and/or calcium alone on building peak bone mass. 120 women will be randomized to one of four groups (i.e. exercise, exercise plus calcium, calcium, and control). Subjects engaging in the exercise portion will do so three days per week for a total of two years. Assessment of bone mineral, fitness and endocrine status will be performed at the outset of the study and every 8 months thereafter. Quantitative techniques such as single photon absorptiometry, dual photon absorptiometry, and quantitative computed tomography will be used to assess skeletal status at multiple appendicular and axial sites. The expected result of this study is a differential skeletal response to the prescribed regimens as compared to control subjects. Given the paucity of information presently available on exercise and calcium in regard to skeletal health, this study will be a much needed contribution to the field. A bank of clinical data will be compiled and analyzed on bone mass, endocrine, and fitness measurements in a young female population and act as a basis for designing future interventions aimed at increasing peak bone mass and improving skeletal status.